Sun sensors are devices used on spacecraft for attitude control thereof. Known sun sensors for attitude control of three axis stabilized and/or spinning spacecraft employ both analog and digital signal processing. Conventional implementation of sun sensors employing analog signal processing include the provision of a slit or pinhole aperture which directs the sun's irradiance on to a quadrature array of silicon photodetectors. By using a radiant balance technique in which the signals from opposite silicon cells are subtracted to result in a difference signal which is related to the off-axis position of the sun, conventional analog sun sensors employ a relatively small number of electronic components, resulting in a relatively low failure rate. However, a sensor output signal indicating the off-axis position of the sun is proportional to the absolute responsivity of the silicon cells, as well as the uniformity of the silicon cell response across its surface. As a result, off-axis accuracy decreases based upon the achievable responsivity stability versus time and environment for large sensor angular fields of view, which are desirable in sun sensor devices. Accordingly, the off-axis accuracy of sun sensors using conventional analog signal processing means exhibit decreased accuracy at relatively wider angular ranges.
Sun sensor devices employing conventional digital signal processing employ silicon detector arrays arranged in an angular code, in which the angular subtense of an element in the array subtends more than the projection of the entrance slit by the sun rays onto the sun sensor array. These known devices provide uniform off-axis accuracy versus time and environment, but the absolute accuracy of such devices is limited to one half of the minimum angular resolution of the Least Significant Bit in the digital silicon detector array.
Conventional sun angle sensor devices including both analog and digital sensor systems are marketed by Adcole Aerospace Products, of Marlborough, Mass. Reference is made to the following publications of Adcole Aerospace Products for further information disclosing known sun sensor devices: "Sun Angle Sensor Systems--Short Form Catalog"; "Spin Mode, V-Slit Sun Sensor Assembly Adco Model 20470"; "Model 18960--Sun Angle Sensor System"; "Model 17470--Analog Sun Sensor"; "Stationkeeping/Yaw Control Sun Sensor Assembly For Geosynchronous Satellites Adco Model 20480"; "Model 16765 Sun Angle Sensor System"; "Model 16764 Sun Angle Sensor System"; "Model 17061 Sun Angle Sensor System"; "Model 18980 Sun Angle Sensor System"; "Model 20910 Sun Angle Sensor System"; and "Model 20020 Sun Angle Sensor System".
It is the primary object of the invention disclosed herein to provide an improved digital sun sensor including the long term accuracy of the conventional digital array sun sensor implementation, but eliminating the absolute accuracy limitation imposed by the relationship between the digital array angular resolution and the angular width of the entrance slit of the sensor. In accordance with this objective, increased accuracy over a wider angular range or field of view is provided in a simplified and economical manner, without employing complicated optical components, as a result of adjustment to the relative positions and dimensions of existing components. The improved arrangement and adjustments are provided in conjunction with improved processing means which increase the overall accuracy of the sun sensor device without the provision of complicated optical components.
Other improvements and advantages of the sun sensor device in accordance with the present invention will become apparent to those skilled in the art from the following discussion.